The present invention relates generally to antenna systems. More specifically, the present invention relates to an improved method and apparatus for providing a shapeable and directable communication beam.
In satellite communication systems, it is desirable to shape and direct communication beams. The ability to shape and direct communication beams results in efficient use of the finite energy resources of communication satellites, increases communication bandwidth, and reduces interference between beams.
In addition, there is a corresponding increase in communication security. Communicating only with an intended geographical area substantially complicates message interception from geographical areas outside the intended area of communication.
In the past, satellite-based phased array antenna systems were developed that provided bandwidth to communication areas using spot beams (communication beams designed to cover specific areas or "spots" on the Earth's surface). Typically the spot beams were organized into a matrix of evenly shaped and spaced beams (also referred to as pixel beams) designed to provide a total coverage to a large geographical area, such as a state, a nation, or the Earth.
The spot beams were generated using conventional phased array antenna systems. In conventional phased array antenna systems, each radiating antenna element in the array has a corresponding independent radio-frequency (RF) phase shifting circuit for each spot beam produced. Thus, for example, in a communication system incorporating a phased array antenna system with 547 elements, 547 corresponding RF phase shifters determine the shape and direction of a single spot beam.
Because of the complexity associated with determining and implementing the large number of RF phase shifts associated with a single spot beam, communication systems typically fixed the shape and direction of the spot beams to predetermined values. The satellite communication system communicates with users in a spot beam area with a corresponding spot beam signal and communicates with users in another spot beam area with another corresponding spot beam signal.
Fixed spot beam communication systems suffer from beam shaping inflexibility. For a fixed spot beam communication system to provide communication bandwidth to an area, the system must provide communication bandwidth to each spot beam area containing a portion of the area. For example, if a desired area includes subsections of three spot beam areas, the system must provide communication bandwidth to the three entire spot beam areas, including the subsections of the three spot beam areas not included in the desired communication area.
Fixed spot beam communication systems also suffer from beam directing inflexibility. A fixed spot beam communication system provides maximum beam gain at the center of each spot beam. Thus, users near the perimeter of spot beam areas receive lower quality communication service than users near the center of spot beam areas. For example, if a desired communication area is centered between three spot beam areas, the system provides maximum quality coverage to the communication area by using all three corresponding spot beams. Unfortunately, in the attempt to provide high quality coverage to the communication area, the system also provides relatively large amounts of communication energy to the centers of the three spot beam areas where the communication energy is not needed or wanted.
A need has long existed for a method and apparatus for forming a shapeable and directable communication beam.